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Cleanup the InstSimplify MIR transformation #139638

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Apr 11, 2025
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Cleanup the InstSimplify MIR transformation #139638

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192 changes: 86 additions & 106 deletions compiler/rustc_mir_transform/src/instsimplify.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -39,26 +39,26 @@ impl<'tcx> crate::MirPass<'tcx> for InstSimplify {
attr::contains_name(tcx.hir_krate_attrs(), sym::rustc_preserve_ub_checks);
for block in body.basic_blocks.as_mut() {
for statement in block.statements.iter_mut() {
match statement.kind {
StatementKind::Assign(box (_place, ref mut rvalue)) => {
if !preserve_ub_checks {
ctx.simplify_ub_check(rvalue);
}
ctx.simplify_bool_cmp(rvalue);
ctx.simplify_ref_deref(rvalue);
ctx.simplify_ptr_aggregate(rvalue);
ctx.simplify_cast(rvalue);
ctx.simplify_repeated_aggregate(rvalue);
ctx.simplify_repeat_once(rvalue);
}
_ => {}
let StatementKind::Assign(box (.., rvalue)) = &mut statement.kind else {
continue;
};

if !preserve_ub_checks {
ctx.simplify_ub_check(rvalue);
}
ctx.simplify_bool_cmp(rvalue);
ctx.simplify_ref_deref(rvalue);
ctx.simplify_ptr_aggregate(rvalue);
ctx.simplify_cast(rvalue);
ctx.simplify_repeated_aggregate(rvalue);
ctx.simplify_repeat_once(rvalue);
}

ctx.simplify_primitive_clone(block.terminator.as_mut().unwrap(), &mut block.statements);
ctx.simplify_intrinsic_assert(block.terminator.as_mut().unwrap());
ctx.simplify_nounwind_call(block.terminator.as_mut().unwrap());
simplify_duplicate_switch_targets(block.terminator.as_mut().unwrap());
let terminator = block.terminator.as_mut().unwrap();
ctx.simplify_primitive_clone(terminator, &mut block.statements);
ctx.simplify_intrinsic_assert(terminator);
ctx.simplify_nounwind_call(terminator);
simplify_duplicate_switch_targets(terminator);
}
}

Expand Down Expand Up @@ -105,43 +105,34 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {

/// Transform boolean comparisons into logical operations.
fn simplify_bool_cmp(&self, rvalue: &mut Rvalue<'tcx>) {
match rvalue {
Rvalue::BinaryOp(op @ (BinOp::Eq | BinOp::Ne), box (a, b)) => {
let new = match (op, self.try_eval_bool(a), self.try_eval_bool(b)) {
// Transform "Eq(a, true)" ==> "a"
(BinOp::Eq, _, Some(true)) => Some(Rvalue::Use(a.clone())),
let Rvalue::BinaryOp(op @ (BinOp::Eq | BinOp::Ne), box (a, b)) = &*rvalue else { return };
*rvalue = match (op, self.try_eval_bool(a), self.try_eval_bool(b)) {
// Transform "Eq(a, true)" ==> "a"
(BinOp::Eq, _, Some(true)) => Rvalue::Use(a.clone()),

// Transform "Ne(a, false)" ==> "a"
(BinOp::Ne, _, Some(false)) => Some(Rvalue::Use(a.clone())),
// Transform "Ne(a, false)" ==> "a"
(BinOp::Ne, _, Some(false)) => Rvalue::Use(a.clone()),

// Transform "Eq(true, b)" ==> "b"
(BinOp::Eq, Some(true), _) => Some(Rvalue::Use(b.clone())),
// Transform "Eq(true, b)" ==> "b"
(BinOp::Eq, Some(true), _) => Rvalue::Use(b.clone()),

// Transform "Ne(false, b)" ==> "b"
(BinOp::Ne, Some(false), _) => Some(Rvalue::Use(b.clone())),
// Transform "Ne(false, b)" ==> "b"
(BinOp::Ne, Some(false), _) => Rvalue::Use(b.clone()),

// Transform "Eq(false, b)" ==> "Not(b)"
(BinOp::Eq, Some(false), _) => Some(Rvalue::UnaryOp(UnOp::Not, b.clone())),
// Transform "Eq(false, b)" ==> "Not(b)"
(BinOp::Eq, Some(false), _) => Rvalue::UnaryOp(UnOp::Not, b.clone()),

// Transform "Ne(true, b)" ==> "Not(b)"
(BinOp::Ne, Some(true), _) => Some(Rvalue::UnaryOp(UnOp::Not, b.clone())),
// Transform "Ne(true, b)" ==> "Not(b)"
(BinOp::Ne, Some(true), _) => Rvalue::UnaryOp(UnOp::Not, b.clone()),

// Transform "Eq(a, false)" ==> "Not(a)"
(BinOp::Eq, _, Some(false)) => Some(Rvalue::UnaryOp(UnOp::Not, a.clone())),
// Transform "Eq(a, false)" ==> "Not(a)"
(BinOp::Eq, _, Some(false)) => Rvalue::UnaryOp(UnOp::Not, a.clone()),

// Transform "Ne(a, true)" ==> "Not(a)"
(BinOp::Ne, _, Some(true)) => Some(Rvalue::UnaryOp(UnOp::Not, a.clone())),

_ => None,
};

if let Some(new) = new {
*rvalue = new;
}
}
// Transform "Ne(a, true)" ==> "Not(a)"
(BinOp::Ne, _, Some(true)) => Rvalue::UnaryOp(UnOp::Not, a.clone()),

_ => {}
}
_ => return,
};
}

fn try_eval_bool(&self, a: &Operand<'_>) -> Option<bool> {
Expand All @@ -151,64 +142,58 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {

/// Transform `&(*a)` ==> `a`.
fn simplify_ref_deref(&self, rvalue: &mut Rvalue<'tcx>) {
if let Rvalue::Ref(_, _, place) | Rvalue::RawPtr(_, place) = rvalue {
if let Some((base, ProjectionElem::Deref)) = place.as_ref().last_projection() {
if rvalue.ty(self.local_decls, self.tcx) != base.ty(self.local_decls, self.tcx).ty {
return;
}

*rvalue = Rvalue::Use(Operand::Copy(Place {
local: base.local,
projection: self.tcx.mk_place_elems(base.projection),
}));
}
if let Rvalue::Ref(_, _, place) | Rvalue::RawPtr(_, place) = rvalue
&& let Some((base, ProjectionElem::Deref)) = place.as_ref().last_projection()
&& rvalue.ty(self.local_decls, self.tcx) == base.ty(self.local_decls, self.tcx).ty
{
*rvalue = Rvalue::Use(Operand::Copy(Place {
local: base.local,
projection: self.tcx.mk_place_elems(base.projection),
}));
}
}

/// Transform `Aggregate(RawPtr, [p, ()])` ==> `Cast(PtrToPtr, p)`.
fn simplify_ptr_aggregate(&self, rvalue: &mut Rvalue<'tcx>) {
if let Rvalue::Aggregate(box AggregateKind::RawPtr(pointee_ty, mutability), fields) = rvalue
&& let meta_ty = fields.raw[1].ty(self.local_decls, self.tcx)
&& meta_ty.is_unit()
{
let meta_ty = fields.raw[1].ty(self.local_decls, self.tcx);
if meta_ty.is_unit() {
// The mutable borrows we're holding prevent printing `rvalue` here
let mut fields = std::mem::take(fields);
let _meta = fields.pop().unwrap();
let data = fields.pop().unwrap();
let ptr_ty = Ty::new_ptr(self.tcx, *pointee_ty, *mutability);
*rvalue = Rvalue::Cast(CastKind::PtrToPtr, data, ptr_ty);
}
// The mutable borrows we're holding prevent printing `rvalue` here
let mut fields = std::mem::take(fields);
let _meta = fields.pop().unwrap();
let data = fields.pop().unwrap();
let ptr_ty = Ty::new_ptr(self.tcx, *pointee_ty, *mutability);
*rvalue = Rvalue::Cast(CastKind::PtrToPtr, data, ptr_ty);
}
}

fn simplify_ub_check(&self, rvalue: &mut Rvalue<'tcx>) {
if let Rvalue::NullaryOp(NullOp::UbChecks, _) = *rvalue {
let const_ = Const::from_bool(self.tcx, self.tcx.sess.ub_checks());
let constant = ConstOperand { span: DUMMY_SP, const_, user_ty: None };
*rvalue = Rvalue::Use(Operand::Constant(Box::new(constant)));
}
let Rvalue::NullaryOp(NullOp::UbChecks, _) = *rvalue else { return };

let const_ = Const::from_bool(self.tcx, self.tcx.sess.ub_checks());
let constant = ConstOperand { span: DUMMY_SP, const_, user_ty: None };
*rvalue = Rvalue::Use(Operand::Constant(Box::new(constant)));
}

fn simplify_cast(&self, rvalue: &mut Rvalue<'tcx>) {
if let Rvalue::Cast(kind, operand, cast_ty) = rvalue {
let operand_ty = operand.ty(self.local_decls, self.tcx);
if operand_ty == *cast_ty {
*rvalue = Rvalue::Use(operand.clone());
} else if *kind == CastKind::Transmute {
// Transmuting an integer to another integer is just a signedness cast
if let (ty::Int(int), ty::Uint(uint)) | (ty::Uint(uint), ty::Int(int)) =
(operand_ty.kind(), cast_ty.kind())
&& int.bit_width() == uint.bit_width()
{
// The width check isn't strictly necessary, as different widths
// are UB and thus we'd be allowed to turn it into a cast anyway.
// But let's keep the UB around for codegen to exploit later.
// (If `CastKind::Transmute` ever becomes *not* UB for mismatched sizes,
// then the width check is necessary for big-endian correctness.)
*kind = CastKind::IntToInt;
return;
}
}
let Rvalue::Cast(kind, operand, cast_ty) = rvalue else { return };

let operand_ty = operand.ty(self.local_decls, self.tcx);
if operand_ty == *cast_ty {
*rvalue = Rvalue::Use(operand.clone());
} else if *kind == CastKind::Transmute
// Transmuting an integer to another integer is just a signedness cast
&& let (ty::Int(int), ty::Uint(uint)) | (ty::Uint(uint), ty::Int(int)) =
(operand_ty.kind(), cast_ty.kind())
&& int.bit_width() == uint.bit_width()
{
// The width check isn't strictly necessary, as different widths
// are UB and thus we'd be allowed to turn it into a cast anyway.
// But let's keep the UB around for codegen to exploit later.
// (If `CastKind::Transmute` ever becomes *not* UB for mismatched sizes,
// then the width check is necessary for big-endian correctness.)
*kind = CastKind::IntToInt;
}
}

Expand Down Expand Up @@ -277,7 +262,7 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
}

fn simplify_nounwind_call(&self, terminator: &mut Terminator<'tcx>) {
let TerminatorKind::Call { func, unwind, .. } = &mut terminator.kind else {
let TerminatorKind::Call { ref func, ref mut unwind, .. } = terminator.kind else {
return;
};

Expand All @@ -290,7 +275,7 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
ty::FnDef(..) => body_ty.fn_sig(self.tcx).abi(),
ty::Closure(..) => ExternAbi::RustCall,
ty::Coroutine(..) => ExternAbi::Rust,
_ => bug!("unexpected body ty: {:?}", body_ty),
_ => bug!("unexpected body ty: {body_ty:?}"),
};

if !layout::fn_can_unwind(self.tcx, Some(def_id), body_abi) {
Expand All @@ -299,23 +284,20 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
}

fn simplify_intrinsic_assert(&self, terminator: &mut Terminator<'tcx>) {
let TerminatorKind::Call { func, target, .. } = &mut terminator.kind else {
return;
};
let Some(target_block) = target else {
let TerminatorKind::Call { ref func, target: ref mut target @ Some(target_block), .. } =
terminator.kind
else {
return;
};
let func_ty = func.ty(self.local_decls, self.tcx);
let Some((intrinsic_name, args)) = resolve_rust_intrinsic(self.tcx, func_ty) else {
return;
};
// The intrinsics we are interested in have one generic parameter
if args.is_empty() {
return;
}
let [arg, ..] = args[..] else { return };

let known_is_valid =
intrinsic_assert_panics(self.tcx, self.typing_env, args[0], intrinsic_name);
intrinsic_assert_panics(self.tcx, self.typing_env, arg, intrinsic_name);
match known_is_valid {
// We don't know the layout or it's not validity assertion at all, don't touch it
None => {}
Expand All @@ -325,7 +307,7 @@ impl<'tcx> InstSimplifyContext<'_, 'tcx> {
}
Some(false) => {
// If we know the assert does not panic, turn the call into a Goto
terminator.kind = TerminatorKind::Goto { target: *target_block };
terminator.kind = TerminatorKind::Goto { target: target_block };
}
}
}
Expand All @@ -346,9 +328,7 @@ fn resolve_rust_intrinsic<'tcx>(
tcx: TyCtxt<'tcx>,
func_ty: Ty<'tcx>,
) -> Option<(Symbol, GenericArgsRef<'tcx>)> {
if let ty::FnDef(def_id, args) = *func_ty.kind() {
let intrinsic = tcx.intrinsic(def_id)?;
return Some((intrinsic.name, args));
}
None
let ty::FnDef(def_id, args) = *func_ty.kind() else { return None };
let intrinsic = tcx.intrinsic(def_id)?;
Some((intrinsic.name, args))
}
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